In QAM data transmission systems, the sequence of data bits to be transmitted is first divided into groups of n bits, wherein each group corresponds to one of 2.sup.n complex numbers or symbols. These symbols are then individually transmitted at T-second intervals, hereinafter called signalling instants. Each symbol is transmitted by causing a given amplitude of each of two carrier waves in quadrature relationship to correspond, respectively, to real and imaginary parts of the complex numbers. The two carrier waves are then combined and applied to the input of the transmission channel.
The transmission channel, connected to a data receiver, provides at its output a signal as similar as possible applied to the transmission channel input signal. However, the transmission channels most often used are telephone lines. The telephone lines, while satisfactory for voice transmission purposes, become less satisfactory when used as data transmission channels for high data speeds equal to or greater than 4800 bits per second (bps) for a particular probability of error in the received data. For high-speed data transmission, the received data is impaired by telephone line limitations. These impairments include amplitude and phase distortions which create an interaction between successive data signal, which make it difficult to correctly detect the transmitted data. This interaction is known as intersymbol interference.
In high-speed data transmission systems, the receiver generally includes an automatic adaptive equalizer to minimize the effects of the intersymbol interference before detection of the data. The type of adaptive equalizer that is the most widely used in those data transmission systems which employ QAM modulation is the complex transversal equalizer, an exemplary embodiment of which is described in U.S. Pat. No. 3,947,768. In such an equalizer, each of the in-phase and quadrature components of the received signal is applied to the inputs of a pair of transversal filters having selectable parameters whose output signals are then combined to generate the in-phase and quadrature components of the equalized signal. The coefficients of these filters, which are generally called coefficients of the equalizer, are automatically adjusted to meet a given performance criterion. Prior to the transmission of the data comprising the actual information to be transferred, it is necessary that the values of the coefficients be as close as possible to optimum values to minimize errors in receiving the data. To this end, provision is made for at least one initial training period during which, before transmitting any information data, a known training sequence of signals is transmitted to the receiver. The receiver compares the received training sequence with an identical, locally generated training sequence. Any differences or errors in the received signals are used by the receiver to optimize the coefficients of the equalizer. In some receivers, the coefficients are then continuously adjusted during transmission of the data.
Moreover, in addition to the equalizer, a modem receiver carrier recovery circuit, AGC circuit, and clock recovery device must all be properly conditioned before the transmission of data begins. Accordingly, provision has been made for a turn-on period, which includes the initial training period. The turn-on sequence is generated by the other transmitter to condition all of the devices contained in the corresponding receiver, including the adaptive equalizer, discussed above. This turn-on sequence generally comprises a first training sequence of signals, also called a preamble, for properly conditioning the AGC circuit and synchronizing the carrier recovery device and the clock recovery device, and is followed by a second equalizer training signal sequence.
In data transmission systems which send many short messages over the same channel, such as half-duplex operations, the turn-on sequence for receiver training occupies a significant portion of the active communication channel time, slowing the transfer of data and reducing the effectiveness of the high-speed data transmission system. Previous efforts directed at minimizing the duration of the turn-on sequence include cyclic equalization schemes of Mueller and Spaulding ("Cyclic Equalization--A New Rapidly Converging Equalization Technique for Synchronous Data Communication," Bell System Telephone Journal, February 1975, pp. 370-406) which achieve turn-on times on the order of tens of milliseconds at significant cost in computational hardware. Other examples of cyclic equalization include U.S. Pat. No. 4,152,649 of Choquet, May 1, 1979; U.S. Pat. No. 4,089,061 of Milewski, May 9, 1978; and U.S. Pat. No. 4,047,013 of Milewski, Sept. 6, 1977. A considerably less powerful computational engine is required to accomplish equalizer training using older, gradient techniques such as the least-mean-squares (LMS) algorithm of Widrow and Hoff (WESCON Convention Record, IRE, 1960, Part 4, pp. 96-104), described in complex form by Proakis and Miller (IEEE, Transactions on Information Theory, Vol. IT-15, No. 4, 1969). Typical turn-on sequences for gradient training techniques include a preamble with strong spectral components at the band edges to allow rapid initial acquisition of receiver timing (clock recovery) relative to the signalling instants, followed by a pseudo-random signal for equalizer training. An example of such a turn-on sequence is the 253-millisecond sequence provided in the CCITT recommendation V.29.
The concept of holding digital filter tap coefficients from one call for use as starting coefficients for echo cancellation of successive calls is shown in U.S. Pat. No. 4,386,430 of Treiber, May 31, 1983.
In systems where the equalizer is adapted to receive signals at 1/T signals per second, wherein T is the signalling instant as mentioned above, the output data signals are provided by the receiver at times nT+.tau., where n=.theta., 1, . . . , and .tau. denotes a timing epoch. Qureshi et al., U.S. Pat. No. 4,004,236, Jan. 18, 1977, noted that .tau. is critical and should be optimized. However, Qureshi et al. use T/2 equalizer tap spacing to desensitize equalizer training to the exact value of the timing epoch .tau.. This allows a shorter turn-on (fast-train) sequence by eliminating the timing recovery segment of the preamble preceding the pseudo-random signal for equalizer training at the expense of doubling the number of tap coefficients to be updated. However, the time thus saved is much less than the remaining time still required for equalizer training.
The strong dependence of 1/T spaced equalizers upon the timing epoch .tau. is well known. Typically, the value of .tau. is optimized by the clock recovery devices which seek to sample at the peak of the envelope of the received signal, thus maximizing the energy of the received signal when sampled at 1/T Hz. The sampling signal is provided by clock recovery devices in the receiver. An example of such a clock recovery device is described by Godard in "Passband Timing Recovery in an All-Digital Modem Receiver" (IEEE, Transactions on Communications, Vol. Com-26, No. 5, May 1978) and U.S. Pat. No. 4,039,748 of Caron et al., Aug. 2, 1977. The optimum value of .tau., according to Godard, is selected during a typical preamble, such as the preamble (segment 2) of the CCITT V.29 turn-on sequence. However, the value of .tau. selected during a typical preamble (with alternating signals) is different from the optimum value of .tau. determined during the subsequent pseudo-random training sequence period, due to differences in frequency content of the respective training sequence periods. If the initial timing epoch .tau. (acquired from an alternating data pattern) is imposed upon a receiver operating with equalizer coefficients trained at a later (revised) timing epoch .tau.' acquired from a subsequent pseudo-random data pattern, the equalizer will fail to completely equalize for intersymbol interference resulting from phone line limitations. The improperly equalized receiver will then operate at an elevated error rate for some period of time until the equalizer can eventually adapt to the new timing epoch. Notwithstanding the recognized critical nature of .tau. with respect to the selection of the equalizer coefficients, no significance has previously been placed on the differences in determination of .tau., nor has there been an accommodation of that difference to allow further modem improvement.